Early Ordovician volcanism in Eucísia and Mateus areas, Central Iberian Zone, northern Portugal
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2012-07-02
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The Eucísia (Alfândega da Fé) and Mateus (Vila Real)areas are located in the Central Iberian Zone, a segment of the northern Gondwana margin which underwent a long Variscan geodynamic evolution. Lithostratigraphically both areas are characterized by a predominance of Precambrian to Cambrian metasedimentary rocks and by the transgressive nature of the Ordovician formations. Continental rifting on the platform of northern Gondwana began around the Middle Cambrian, and opening of the Rheic ocean took place near the Ordovician-Cambrian boundary. In the Eucísia and Mateus areas there is evidence of extensional volcanism related to these events,contemporaneous with a transient inversion, known as the Sardic phase. At Eucísia the Ordovician Armorican Quartzite Formation hosts a < 40 cm thick ash-fall tuff bed, which is mainly composed by muscovite and small amounts of quartz. At Mateus, a subvolcanic porphyritic rock occurs in a 3 m thick vein cutting Precambrian-Cambrian schists. The phenocrysts consist of euhedral/ovoidal plagioclase and microcline and rounded bluish quartz. The groundmass is dark, fine grained and contains quartz, feldspars, muscovite, rare biotite and graphite (identified by XRD). The mineralogical and textural features of this vein resemble those of the augen gneisses from the Ollo de Sapo Formation in Sanabria (Spain), except for a lower degree of deformation/recrystallization. The tuff and the porphyry are both alkali-calcic, peraluminous (ASI ' 1.75 – 2.91) and classified as rhyolites in the R1-R2 diagram. They show moderate REE contents (I ' 78.3 – 182.6), (La/Lu)N values between 11.78 and 12.06, weak negative Eu anomalies (Eu/Eu* ' 0.45 – 0.52) and REE patterns typical of peraluminous granites. Both rocks have a high content of zircon xenocrysts, but also some magmatic zircon prisms which were dated by U-Pb (ID-TIMS) yielding concordia ages that indicate crystallization in the Early Ordovician: the ashfall tuff from Eucísia at 482.1 ± 1.5 Ma and the rhyolite porphyry from Mateus at 478.0 ± 1.7 Ma.
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Early Ordovician volcanism , Central Iberian Zone